The present application is a continuation-in-part of application Ser. No. 09/881,561, filed Jun. 14, 2001 now abandoned, which claims priority from Provisional Application Ser. No. 60/212,066, filed Jun. 14, 2000, both of which are incorporated herein by reference for all purposes.
This invention relates generally to battery charging systems and methods for charging batteries and, more particularly, to a cost-efficient charging system and method for simultaneously charging the batteries of a plurality of electrically powered vehicles such as forklifts.
Increasing numbers of vehicles (e.g., forklifts) are being manufactured as electric vehicles. Vehicle charging systems for the batteries of larger numbers of vehicles, such as for a fleet of forklifts vehicles, are therefor increasingly important. The implementation of such charging systems in existing facilities' electrical systems (e.g., building electrical systems) presents significant problems that can lead to large capital expenditures, as described below.
Parallel Charging
Facilities' electrical systems are typically formed in a multi-level, branched architecture. At each branching level, a plurality of receiving circuit breakers draws current from a distributing circuit breaker, which must have a current capacity equal to (or greater than) the sum of those of the circuit breakers that it distributes to. Each of the receiving circuit breakers in turn act as distributing circuit breakers to other circuit breakers till the end of each branch, i.e., a load such as a charging system, is reached. Because the electrical system power typically originates from an AC source, a load requiring DC power, such as a battery charger, will typically require an AC rectifier upstream from the load.
As shown in FIG. 1, an existing charging system, will typically include a system/utility circuit breaker (CB1) connected in series with a number of vehicle chargers, each of which has its own associated circuit breaker (CB2 and CB3). Each charger can charge one vehicle at a time (vehicle #1 or #2), and can operate at any current up to the limit of its associated circuit breaker. Typically the system circuit breaker has the capacity to operate at a current level up to the sum of each of the charger's circuit breakers, so the current limit of the system circuit breaker CB1 will be at or slightly over the sum of the existing associated circuit breakers CB2 and CB3.
Upgrading such a charging system to charge greater numbers of vehicles (or installing a battery charging system where none is in place) can significantly increase the current carried by the system circuit breaker, and therefore it will likely need to be upgraded to carry additional current. Increasing the maximum current capacity of the system circuit breaker (and related power transmission equipment) that supports the battery charging system requires increased capacity in each distributing circuit breaker upstream (along the circuit) from that system circuit breaker. Thus, increasing the number of vehicles that can be charged can potentially require expensive upgrading of a substantial portion of the facility's electrical system, requiring significant capital expenditures.
For example, as depicted in FIG. 1, in order to simultaneously charge additional vehicles (#4, #5 and #6), additional circuit breakers (CB4, CB5 and CB6), additional chargers and additional wiring are added to the system. The addition of these new circuits to the system requires that all name plate ratings of charging circuit breakers (CB2 to CB6) be added up to establish a new current value that the rating of the system wiring or of the system circuit breaker (CB1) cannot exceed. This is required even though the individual chargers might not all be in use at the same time and, if they are in use, they will most likely not be simultaneously operating at full power and fully utilizing the existing infrastructure. Not only will the system circuit breaker (CB1) need upgrading, but many or all of the upstream circuit breakers will need to be upgraded to support the system circuit breaker's (CB1) additional capacity. Thus, the capital expense of adding vehicle chargers to a system potentially includes the significant costs of upgrading a significant portion of the entire electrical system.
Furthermore, the batteries in each vehicle will likely have different charging requirements. For example, in FIG. 1 vehicle #1 might only need a low current for equalization, while vehicle #2 might need a larger current for fast-charging. While the chargers can be configured to handle either load level, the capacity of the charger used on vehicle #1 will be wasted even though the facility's entire electrical system was rebuilt to support the larger load.
As a result, the capital investments necessary to provide new or increased battery charging systems do not have an efficient, high and/or maximum rate of return. Additionally, where significant additional installations of battery chargers are desired, major costs might be incurred to upgrade a facility's electrical system even though the fundamental level of power available in the building is sufficient to supply the total kW hrs of power needed.
Sequential Chargers
One known approach to this problem is to install sequential chargers. Sequential chargers utilize charge capability in an improved, but not especially efficient, manner. Sequential chargers use a set of switches to connect a single charger to a series of vehicles.
As depicted in FIG. 2, with sequential chargers, additional vehicles can be added to an existing system without the need for additional current, and thus, without upgrading the entire electrical system. However, only one vehicle can be charged at a time in such a system. To the degree that this fully utilizes the facility's installed electrical system capacity for that branch, this reaches optimum usage during a normal battery charge event. However, during a typical battery charge cycle the amount of delivered current drops as the battery is more fully charged. Thus the charger will at best only fully utilize the utility during the initial stages of charging. Furthermore, where the battery charging is not a maximum level for reasons related to accommodating battery life characteristics, such as the battery voltage, charge acceptance, and optimum power, a non-optimal level will be achieved. For example temperature may limit the charge rate, SOC may limit the charge rate, or the battery charger current limit may limit the charge rate, such as when a 60 volt capable charger charges a 24 volt battery at the same current, providing a much lower power requirement.
Additionally, the contactors and wiring of the sequential chargers are large. To the degree that a certain number of vehicles must be charged in a given time, the charger capacity must be increased by a minimum of the number of vehicles. This further aggravates the first problem as a larger charger is further underutilized, and the charger's components all are larger to accommodate the higher charge rate.
Accordingly, there has existed a definite need for a cost-efficient charging system and a method for simultaneously charging a plurality of vehicles. The present invention satisfies these and other needs, and provides further related advantages.